This invention relates to a semiconductor memory devices and more particularly to an improved sense amplifier for an electrically-erasable, electrically-programmable, read-only-memory (EEPROM) memory.
An EEPROM memory cell typically comprises a floating-gate field-effect transistor. The floating-gate conductor of a programmed memory cell is charged with electrons, and the electrons in turn render the source-drain path under the charged floating gate nonconductive when a predetermined voltage is applied to the control gate. The nonconductive state is read by a sense amplifier as a "zero" bit. The floating-gate conductor of a non-programmed cell is neutrally charged (or slightly positively or negatively charged) such that the source-drain path under the non-programmed floating gate is conductive when the predetermined voltage is applied to the control gate. The conductive state is read by a sense amplifier as a "one" bit.
Each column and row of an EEPROM array may contain thousands of floating-gate memory cells. The sources of each cell in a column are connected to a source-column line and the source-column line for a selected cell may be connected to reference potential or ground during reading of the selected cell by a sense amplifier. The drains of each cell in a column are connected to a separate bitline (drain-column line) and the drain-column line for a selected cell is connected to the input of the sense amplifier during reading of the selected cell. The control gates of each cell in a row are connected to a wordline, and the wordline for a selected cell is connected to the predetermined select voltage during reading of the selected cell.
Prior art sense amplifiers attempt to read the state of a selected memory cell by comparing the current of the selected memory cell to a reference cell current. While comparing the currents, the amplifier attempts to maintain a constant voltage on the drain-column line. However, noise can cause small voltage changes on the drain-column line, thus causing erroneous current readings. Therefore, the designer has to choose between increased amplifier gain or decreased sensitivity to noise.
The invention overcomes the above-noted and other deficiencies of the prior art by providing a method and apparatus for providing drain column voltage swings during current sensing. The invention features the capability of purposefully producing, on the column-drain line, voltages above and below a reference voltage in response to a reading of the state of the memory cell. As a result, the sense amplifier of the present invention achieves both increased gain and decreased noise sensitivity. In the sense amplifier the output of the sense amplifier is fed back to bias transistors in parallel with sensing transistors, to reinforce the output by positive feedback.